Nowadays, ethylene oxide is produced by catalytic gas phase oxidation of ethylene using a molecular oxygen-containing gas in the presence of a silver catalyst. An outline of a purifying method in a process for producing ethylene oxide is as follows (for example, refer to JP 62-103072 A).
First, ethylene and a molecular oxygen-containing gas are subjected to catalytic gas phase oxidation on a silver catalyst to obtain an ethylene oxide-containing reaction product gas (reaction step). Subsequently, the resulting reaction product gas is introduced into an ethylene oxide absorption column. The reaction product gas is brought into contact with an absorption liquid mainly containing water. Ethylene oxide is recovered as an aqueous solution (absorption step). Subsequently, the recovered ethylene oxide aqueous solution is fed to a purification system of ethylene oxide to obtain high-purity ethylene oxide through several stages. The ethylene oxide purification system usually includes a stripping step, a dehydration step, a light fraction separation step, a heavy fraction separation (purification) step, and the like.
Usually, an exhaust gas discharged from a column top part of the ethylene oxide absorption column, which contains unreacted ethylene, carbon dioxide and water as by-products, and an inert gas (nitrogen, argon, methane, ethane, or the like) is circulated into an ethylene oxidation step as it is. Alternatively, a part thereof is extracted and introduced into a carbon dioxide absorption column, the carbon dioxide is selectively absorbed by an alkali absorption liquid and the carbon dioxide is stripped and recovered from the absorption liquid.
Meanwhile, the molecular oxygen-containing gas, which is a reaction raw material, contains argon. As described above, when the exhaust gas from a column top part of the ethylene oxide absorption column is simply circulated in the ethylene oxidation step or the carbon dioxide absorption step, argon is accumulated in the process circulating gas. When argon is accumulated in the process circulating gas, the pressure of the reaction system may increase, making operation at constant pressure impossible. In addition, when the concentration of argon in the process circulating gas becomes too high, due to decreases in the ethylene concentration and the oxygen concentration, the EO reaction rate may decrease. Thus, usually, the exhaust gas from the column top part of the ethylene oxide absorption column is partially extracted and purged (argon purging).
As described above, the exhaust gas from the column top part of the absorption column (absorption column exhaust gas) to be purged with argon contains ethylene, which is a raw material for the production of ethylene oxide, and the amount of ethylene loss caused by argon purging is as much as 0.55% of the amount of ethylene fed to the ethylene oxidation reactor. Therefore, it has been demanded to reduce the argon purge amount.